System and method for a rangefinding instrument incorporating pulse and continuous wave signal generating and processing techniques for increased distance measurement accuracy

ABSTRACT

A system and method for a rangefinding instrument incorporating pulse and continuous wave signal generating and processing techniques for increased distance measurement accuracy. The use of the former technique effectively solves the ambiguity issues inherent in the latter while allowing for relatively simple circuit implementations. Thus, a potentially more accurate phase-based distance measurement technique can be utilized which is also completely independent of the maximum range to the target.

CROSS REFERENCE TO RELATED PATENT APPLICATIONS

The present invention claims priority from, and is a continuation ofU.S. patent application Ser. No. 13/615,143 filed on Sep. 13, 2012, andis related to the subject matter disclosed in U.S. patent applicationSer. No. 13/615,172 for: “System and Method for Superimposing a VirtualAiming Mechanism with a Projected System Beam in a Compact Laser-BasedRangefinding Instrument” and Ser. No. 13/615,215 for: “Self-AlignedAiming System and Technique for a Laser Rangefinder Incorporating aRetroreflector” both assigned to the assignees hereof and filed on evendate herewith, the disclosures of which are herein specificallyincorporated by this reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates, in general, to the field of laserrangefinders and rangefinding instruments. More particularly, thepresent invention relates to a system and method for a rangefindinginstrument incorporating pulse and continuous wave signal generating andprocessing techniques for increased distance measurement accuracy.

Numerous types of ranging devices have been introduced for measuringdistances in situations in which traditional tape measures havepreviously been employed. For such applications, compact design and costconsiderations are important factors in addition to ease of use andoverall device accuracy.

Certain of these designs are pulse-based laser systems which utilize atime-of-flight measurement technique to compute the distance to aparticular target based on the time it takes for a transmitted pulse toreach the target and be reflected back therefrom. Among thedisadvantages of such pulse-based devices is that, in order to achievehigh levels of accuracy, rather complicated timing circuitry is requiredin addition to ensuring that the device remains properly calibrated forthe then current operating conditions. Representative circuitry andassociated signal processing techniques for such a pulse-based laserrangefinder are disclosed, for example, in the following United StatesPatents assigned to Laser Technology, Inc., assignee of the presentinvention: U.S. Pat. Nos. 5,574,552; 5,612,779; 5,652,651; 5,703,678;5,880,821; 6,057,910; 6,226,077 and 6,445,444. The disclosures of thesepatents are herein specifically incorporated by this reference in theirentirety.

An alternative technique for measuring distances employs phase-basedtechniques in which a continuous wave (CW) or discreet bursts ofessentially continuous waves signals are directed toward a target andthe phase of the backscatter signal that is reflected back therefrom isdetermined. Among the advantages of such CW systems is that, for similaror lower costs than a pulse-based system, it is possible to achievepotentially higher accuracy with simpler electronics due, at least inpart, to the fact that the small CW diodes will turn “on” and “off” morequickly than the larger infrared (IR) pulse diodes used in pulse-basedsystems.

However, among the difficulties inherent in implementations of suchphase-based systems is that the unambiguous range of the instrument isthe period of the CW signal divided by two. In other words, assuming a50 MHz signal which has a cycle time of 20 nsec., a flight distance ofabout 10 feet is covered so it is not possible to discriminate based onthat information alone whether the target is at 10 feet, 20 feet, 30feet or the like. Known techniques for dealing with this ambiguityinclude generating and transmitting multiple frequencies or dividingdown the device clock frequency to produce varying transmissionfrequencies. All of these solutions require ever more complicatedcircuitry and the problem they attempt to solve becomes increasinglymore difficult to address as the distance from the target increases.

A comparison of pulse-based systems with phase-based systems shows thatthe former can effectively distinguish between dust or other debris orinterference between the signal source and the intended target. In otherwords, a pulse-based system can distinguish between multiple targets. Onthe other hand, in phase-based systems the phase return is the vectorsum of the backscatter from the target and the interfering dust or otherobject and there is no easy way of discriminating between the two.

Therefore, a need exists for a compact, low cost rangefinding instrumentwhich is inexpensive, reliable and highly accurate and would essentiallyprovide the benefits of both pulse-based and phase-based ranging systemswhile compensating for, or overcoming, the inherent disadvantages of theother.

SUMMARY OF THE INVENTION

Disclosed herein is a system and method for a rangefinding instrumentincorporating pulse and continuous wave signal generating and processingtechniques for increased distance measurement accuracy. The use of theformer technique effectively solves the ambiguity issues inherent in thelatter while allowing for relatively simple circuit implementations.Thus, a potentially more accurate phase-based distance measurementtechnique can be utilized which is also completely independent of themaximum range to the target. Moreover, under conditions in which thetarget cannot be discriminated from any interfering dust or other objectwith a phase-based ranging technique, the pulse-based system can beemployed and a warning flagged to the user as to the then currentaccuracy of the phase-based system.

Essentially then, one system can be used to calibrate the other and tocompensate for the inherent disadvantages of the other. Other advantagesof the system and method of the present invention which employs apulse-based as well as a phase-based distance measuring technique aremore fully disclosed and claimed in the aforementioned co-pending UnitedStates Patent Application for “System and Method for Superimposing aVirtual Aiming Mechanism with a Projected System Beam in a CompactLaser-Based Rangefinding Instrument”.

Particularly disclosed herein is a rangefinding instrument whichcomprises a pulse-based signal transmission system, a phase-based signaltransmission system and a distance computing circuit coupled to thepulse-based and phase-based transmission systems which is operative todetermine a range to a target based upon a time of flight of thepulse-based signals and a phase shift of the phase-based signalsreflected from the target.

Also particularly disclosed herein is a method for determining range toa target which comprises transmitting a pulse-based signal to thetarget, transmitting a phase-based signal to the target and determininga range to the target based upon a time of flight of the pulse-basedsignal and a phase shift of the phase-based signal reflected from thetarget.

Further disclosed herein is a method for measuring distance comprisingemitting a continuous wave laser beam toward a target, emitting a pulsedlaser beam toward the target and receiving reflections of the continuouswave laser beam and the pulsed laser beam from said target. Distance tothe target is then computed based on the emitted and reflectedcontinuous wave and pulsed laser beams and the computed distance basedon the pulsed laser beam is utilized to resolve phase ambiguity in thecomputed distance based on said continuous wave laser beam.

Still further provided herein is a method for measuring a distancecomprising emitting a continuous wave laser beam toward a target, alsoemitting a pulsed laser beam toward the target and utilizing both thecontinuous wave and pulsed laser beams to determine the distance to thetarget.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned and other features and objects of the presentinvention and the manner of attaining them will become more apparent andthe invention itself will be best understood by reference to thefollowing description of a preferred embodiment taken in conjunctionwith the accompanying drawings, wherein:

FIG. 1 is a simplified illustration of the optical and signal paths in arepresentative embodiment of a possible implementation of the system andmethod of the present invention for a rangefinding instrumentincorporating pulse and continuous wave signal generating and processingtechniques for increased distance measurement accuracy;

FIG. 2 is an isometric view of the representative embodiment of theinstrument of the preceding figure illustrating, for example, a displayand data input key pad;

FIG. 3 is a functional block diagram of a possible embodiment ofcircuitry for implementing the pulse and continuous wave signalgenerating and processing techniques of the present invention;

FIG. 4 is a functional block diagram of another possible embodiment ofcircuitry for implementing the pulse and continuous wave signalgenerating and processing techniques of the present invention;

FIGS. 5A through 5G inclusive illustrate greater structural detail ofthe representative device of the preceding figure including portions ofthe pulse and continuous wave generating portions;

FIGS. 6A through 6J inclusive illustrate greater structural detail ofthe representative device of the preceding figures inclusive of themicrocontroller and associated circuitry; and

FIGS. 7A through 7E inclusive illustrate further structural detail ofthe representative device of the preceding figures inclusive of portionsof the device power supply and associated circuitry.

DESCRIPTION OF A REPRESENTATIVE EMBODIMENT

With reference now to FIG. 1, a simplified illustration of the opticaland signal paths of a representative embodiment of the system and methodof the present invention is shown in the form of a pulse-based andphase-based rangefinding instrument 100. The system and method of thepresent invention is operative to increase distance measurement accuracyin a rangefinding instrument by incorporating pulse and continuous wavesignal generating and processing techniques.

The instrument 100, in pertinent part, comprises a pulsed laser beam andphase-based (CW) optical signal transmission aperture 102 and areflected pulsed laser beam and continuous wave signal receptionaperture 104, the latter for reception of both pulse-based andphase-based signals transmitted to a selected target from thetransmission aperture 102 and reflected back to the instrument 100 froma target. The laser transmission system of the instrument 100 comprisesa laser emitting diode 106 (or other similar signal producing mechanism)and a collimating lens 108. Laser light, typically infrared, is passedthrough the lens 108 to a mirror 110 whereupon it is redirected 90°toward the selected target through the laser transmission aperture 102in the form of a laser beam 112. As previously described, laser lightreflected from the target is received through the reception aperture 104as well as lens 114 to be focused on a signal detection diode 116 orsimilar device comprising a part of the pulse-based and phase-basedreceiving system of the instrument 100.

The instrument 100 also comprises a phase-based, continuous wave,visible light source 118 which is positioned adjacent a collimating lens120 for directing light towards a partial (e.g. dichroic) mirror 122positioned substantially as shown to redirect incident light 90° along abeam path 124 substantially coaxially with the laser beam 112. In theparticular implementation shown, approximately 95% of the visible lightincident upon the mirror 122 is reflected towards the target along beampath 124 while about 4% is passed through the dichroic 122 mirrortowards a reflective surface 128 along beam path 126. Of theapproximately 4% of the visible light incident upon the reflectivesurface 128 something on the order of about 0.2% is then redirected backtowards the other side of the dichroic mirror 122 and then redirected90° towards a viewer/user of the instrument 100 along path 130 andthrough viewing aperture 132.

Power for the electronic portions of the transmission, reception andother systems of the instrument 100 may be conveniently provided by abattery 134 as shown. A distance computing circuit 136 is operativelycoupled to the pulse-based and phase-based signal transmission sectionincluding the laser emitting diode 106 and light source 118 as well as apulse-based and phase-based beam reception section including signaldetection diode 116 to calculate the distance to the target as will bemore fully described hereinafter.

With reference additionally now to FIG. 2, an isometric view of therepresentative embodiment of the instrument 100 in the preceding figureis shown. Like structure to that previously shown and described withrespect to the preceding figure is like numbered and the foregoingdescription thereof shall suffice herefor.

The instrument 100 comprises a hand held housing 200 for containing thelaser, optical and electronic systems previously described as well as auser actuatable switch 202. A display 204 may be utilized to indicaterange, operational mode or other pertinent data to a user who can alsoenter data to the instrument 100 through, for example, a keypad 206. Thedisplay 204 and keypad 206 are also operatively coupled to the distancecomputing circuit 136.

With reference additionally now to FIG. 3, a functional block diagram ofa possible implementation of distance computing circuit 136 (FIG. 1) inthe form of a circuit 300 for implementing the pulse and continuous wavesignal generating and processing techniques of the present invention isshown. The circuit 300 comprises an oscillator 302 which may provide anoutput signal with a frequency of, for example, substantially 100 MHz toa synchronizer block 304 as well as the clock (CLK) input of a counter306. The synchronizer block 304 further provides a START signal input tothe counter 306.

The 100 MHz output signal from the oscillator 302 is also input to adivide-by-two function 308 to reduce the frequency to substantially 50MHz for input to a continuous wave transmit (CW TX) signal emitter 310and a phase shifter 332. The continuous wave signal emitter generallycorresponds with the visible light source 118 of FIG. 1 and the signalemitted may comprise visible light. The START signal output of thesynchronizer block 304 is also supplied to a pulse signal emitter 312which may comprise an infrared (IR) laser light emitting diode generallycorresponding to laser emitting diode 106 of FIG. 1.

The counter 306 provides flight time (count data) information to acentral processing unit (CPU) 314 which, in turn, provides back a countreset signal to the counter 306. The central processing unit 314provides a control signal to the synchronizer block 304 as well as aFIRE signal to the CW TX signal emitter 310. In addition, the centralprocessing unit 314 provides an input to the phase shifter 332 as wellas a digital signal input to a digital-to-analog converter (DAC) 316.Output of the DAC 316 is provided as a threshold voltage V_(THRESH) tothe “−” input of a comparator 318. The “+” input of the comparator 318receives a PULSE signal input from a CW/PULSE Receiver (RX) 320 whichsenses the CW and PULSE signals emitted by emitters 310 and 312 asreflected back from a target. The PULSE signal is also applied to amixer 324 which is also coupled to the output of the phase shifter 330.

Output of the mixer 324 is passed through a low pass filter 326 forinput to an analog-to-digital converter (ADC) 328 for application of adigital signal to the central processing unit 314. The digital signalcomprises digital data representative of the phase angle of the signalfrom the target and allows adjustment of the comparator 318 threshold inorder to optimize the detector sensitivity. Output of the comparator 318in the form of a digital pulse is furnished to the counter 306 in theform of a STOP signal as well as to a resistive/capacitive network 330coupled to the ADC 328.

In operation, the pulse-based system of the circuit 100 fires offhigh-intensity pulses (typically IR) of short duration (typically a fewnanoseconds) toward a target and then measures the direct time of flightof a particular pulse from the target. This provides a method ofmeasuring the flight time of the pulse, and hence, determining the rangeto the target without any ambiguity. As mentioned previously, thedownside of such a system is that in order to achieve high levels ofaccuracy, relatively complex circuitry and precise calibration isrequired.

With phase-based systems, lower cost circuitry can employed along withthe possibility of achieving higher accuracy in the measurement ofdistance as the linear operating range is a much broader range ofdistances than that of pulse-based systems. With the phase-based portionof the circuit 300, a continuous wave, or discreet bursts of essentiallycontinuous wave signals, is directed to the target. At a 50 MHzoperating frequency out of the divide-by-two function 308, a discreetseries of CW pulses is output from the CW TX signal emitter 310 andgated “on” and “off” under control of the CPU 314. These signals willgenerally have a 50% duty cycle to minimize errors in the return pathwhich results in being “on” for about 10 nsec. and “off” for the samelength of time so the duration of the wave-train is relatively longcompared to the spacing between the individual pulses that are beingemitted. In comparison, the output of the pulse signal emitter 312 is aseries of pulses of approximately 10 nsec. duration emitted every fewmilliseconds.

Reflected pulses from the target are detected by the circuit 300 whenthey exceed a determined threshold and time of flight is measured bystarting the counter 306 at the exact moment a pulse is fired from thepulse signal emitter 312 and stopping it at the precise moment when thecorresponding reflected pulse is received. The resultant count stored inthe counter 306 is then provided to the central processing unit 314 tocompute distance to the target. By controlling the threshold, any noisein the returned signal can be integrated to optimize the sensitivity ofthe system.

With the phase-based portion of the circuit 300, a continuous wavesignal is directed to the target and that same signal is reflected backto the instrument with a certain determinable phase shift caused by thetime delay to and from the target. The circuit 300 mixes the returnsignal from the target using mixer 324 with a phase-shifted version ofthe signal that was emitted from the CW TX signal emitter 310 at theoutput of the phase shifter 332. That is then applied through the lowpass filter 326 to provide a voltage to the input of the ADC 328. Inthis manner, both the amplitude and phase of the received signal can bedetermined relative to the transmitted signal.

Functionally, an instrument incorporating the features of circuit 300can determine from the pulsed signal data whether or not a confusedtarget has been encountered due to intervening dust or other inferringobjects, should multiple pulses be encountered. If multiple pulses arenot encountered, an assumption can be made that there is a clean line ofsight to the target and back. Under those conditions, the more accuratephase measurement technique can be employed to calibrate pulse rangesuch that when the target does become intermittent, accuracy ismaintained on the pulse measurement because the range accuracy of thepulse-based system has been improved by the prior measurement of thephase-based system. In other words, the pulse-based system serves as amonitor for a “clean” target, and when such is encountered, it canupdate its calibration tables from the more accurate phase measurement.

With reference additionally now to FIG. 4, an alternative representativeembodiment of a laser distance measuring device 400 in accordance withthe present invention is shown. The device 400 comprises an MCU 402with, for example, an on board digital-to-analog converter (DAC) whichreceives clocking signals from clock source 404 as shown. The MCU 402provides a number of control inputs, herein collectively labeled asPLL_CNTL, to a fractional N phase-locked loop (PLL) synthesis circuit406 as will be shown and described more fully hereinafter. Thefractional N PLL synthesis circuit 406 provides output frequencies of230.0 MHz and 230.01 MHz, the latter being at a 10.0 KHz differentialwith respect to the former.

The 230.0 MHz frequency signal is supplied as one input to a switch 408which also receives a PHASE_TX/REF signal from the MCU 402. Outputs ofthe switch 408 are supplied to identical phase reference and phasetransmission blocks 410, 412 as will also be described more fullyhereinafter. It should be noted that in other embodiments of the presentinvention, the presence of the switch 408 may be obviated through theuse, for example, of the Y4 and Y5 outputs of the fractional N PLLsynthesis circuit 406 when a CDCE925/CDCEL925 device available fromTexas Instruments, Inc. is utilized. In this instance, the Y4 output canbe supplied to the phase reference block 410 and the Y5 output can besupplied to the phase transmission block 412.

The phase reference block 410 receives a number of control inputs,herein collectively labeled as PH_REF CNTL, from the MCU 402 which alsoprovides a number of control inputs, herein collectively labeled asPH_TX CNTL, to the phase transmission block 412. The phase referenceblock 410 provides signal directly to the laser receiver 414 as shown.The phase transmission block 412 emits a continuous wave laser signaltoward a target from which a return signal is subsequently received bythe laser receiver 414.

The laser receiver 414 provides an RX signal to other of the device 400circuitry including a mixer 416 which also receives the 230.01 MHzoutput signal from the fractional N PLL synthesis circuit 406 and aPULSE/PHASE signal from the MCU 402. Output from the mixer 416 is passedthrough a low pass filter (LPF) network 418 to the MCU 402.

The clock source 404 also provides a clocking signal to a series offlip-flops 424, 426 and 428 which will be illustrated and described inmore detail hereinafter. The delay circuit 420 receives a V_DEL signalfrom the MCU 402 and provides an output to a demultiplexer 422 as shownwhich also receives a transmission/calibration signal TX/CAL from theMCU 402. The series of flip-flops 424, 426 and 428 receive signals fromthe MCU 402 denominated as TX_MATCH, REF_MATCH and RX_MATCHrespectively. Output from flip-flop 424 is supplied to the delay circuit420 while output of flip-flop 426 is supplied as one input to a samplinggate 430 which also receives an output from the demultiplexer 422 and aTX REF signal. Output from flip-flop 428 is supplied as one input tosampling gate 432 which is also coupled to an output of thedemultiplexer 422 and also receives a RX signal from the receiver 414.The demultiplexer 422 provides a driving signal to a pulse transmissionblock 448 which may function, for example, as disclosed in U.S. Pat.Nos. 5,612,779; 6,057,910 and 6,226,077 assigned to Laser Technology,Inc., assignee of the present invention, the disclosures of which areherein incorporated by this reference in their entirety.

Activation of the device 400 is accomplished by a user aiming it at atarget and activating a fire button 434. Additional associated devicesand circuitry for the device 400 include a keypad 436, an inclinometer438 and rate gyro 440 as well as a serial port 442 and serial wire debug(SWD) port 444. The inclinometer, or tilt sensor, 438 may beconveniently furnished as an accelerometer as will be more fullydescribed hereinafter. A universal serial bus (USB) port 450 may also becoupled to the MCU as indicated. Information regarding distance to atarget point as well as other user information such as battery status,head-up display, aiming reticule, operational mode and the like may bedisplayed in a liquid crystal display (LCD) 446.

With reference additionally now to FIGS. 5A through 5G inclusive,greater structural detail of the representative device 400 of thepreceding figure is shown. With respect to FIG. 5A in particular,elements of the delay circuit 420 and demultiplexer 422 are shown. Theactive components of the delay circuit 420 and associated circuitrycomprise small signal NPN transistors 502 and 506 which may beconveniently furnished as MMBT3904 devices while transistor 504 may befurnished as a small signal PNP transistor such as an MMBT3906 device.Both devices are available from Diodes, Inc. Demultiplexer 422 maycomprise an SN74LVC1G19 1-of-2 decoder/demultiplexer 510 while inverter512 may be an SN74LVC1G04 single inverter gate. Both devices areavailable from Texas Instruments, Inc.

With reference particularly to FIG. 5B, the circuitry associated withflip-flops 424, 426 and 428 as well as sampling gate 430 (FIG. 4) areshown. Flip-flops 424, 426 and 428 may comprise SN74LVC1G79 singlepositive-edge-triggered D-type flip-flops while buffer 516 may comprisean SN74AUC1G17 single Schmitt-trigger buffer. Sampling gate, or trackand hold circuit, 430 may also conveniently comprise an SN74AUC2G53single-pole double-throw (SPDT) analog switch or 2:1 analogmultiplexer/demultiplexer. All of these devices are available from TexasInstruments, Inc. The storage capacitor coupled between the COM outputof the sampling gate 430 and circuit ground is operative with a CMOSoperational amplifier (or field effect transistor) 514 to provide avoltage REF_SMPL the instant the switch opens.

Referring additionally now to FIG. 5C the circuitry associated withsampling gate 432 (FIG. 4) is shown in greater detail. Sampling gate 432may be furnished as an SN74AUC2G53 device as sampling gate 430 of thepreceding figure. Operational amplifier (op amp) 518, as op amp 514, maybe an MCP6284 rail-to-rail op amp available from Microchip Technology,Inc. while transistor 520 may be furnished as a DMP216 P-channelenhancement mode MOSFET available from Diodes, Inc. As with thepreceding figure, the storage capacitor coupled between the COM outputof the sampling gate 432 and circuit ground is operative with a CMOSoperational amplifier (or field effect transistor) 518 to provide avoltage RX_SMPL the instant the switch opens.

With particular reference now to FIG. 5D, the clock source 404 andfractional N PLL synthesis circuit 406 (FIG. 4) and associated circuitryare shown in greater detail. The clock source 404 comprises, inpertinent part, an oscillator chip 522 which may be furnished as anASFL3 1.8v LVHCMOS compatible SMD crystal clock oscillator availablefrom Abracon Corporation while the associated transceiver 524 maycomprise an SN74AVC1T45 single-bit dual-supply bus transceiver availablefrom Texas Instruments, Inc. Transistor 526 may also be a DMP216P-channel enhancement mode MOSFET available from Diodes Inc.

Referring now to FIG. 5E, the mixer 416 and low pass filter (LPF) 418(FIG. 4) are shown in greater detail along with some of their associatedcircuitry. The mixer 416 may comprise an SN74AUG2G53 SPDT analog switchas sampling gates 430 and 432. Op amps 528 and 530 may both beconveniently furnished as MCP6284 devices from Microchip Technology,Inc.

With reference additionally now to FIG. 5F, the switch 408 and phasereference block 410 are shown in greater detail. As before, switch 408may be furnished as an SN74AUG2G53 device. The phase reference block 410comprises a laser light emitting diode 532 _(REF) while transistors 534_(REF) and 544 _(REF) may comprise MMDT3946 complementary NPN/PNP smallsignal transistors available from Diodes, Inc. Inverters 536 _(REF), 538_(REF) and 540 _(REF) may be furnished as portions of SN74LVC2G14 dualSchmitt-trigger inverters available from Texas Instruments, Inc.Amplifier 542 _(REF) may be provided as an FAN4931 rail-to-rail I/O CMOSamplifier device available from Fairchild Semiconductor Corp.

Referring additionally to FIG. 5G, the phase transmission block 412(FIG. 4) is also shown in greater detail. The phase transmission block412 is functionally identical to the phase reference block 410. Thephase transmission block 412 comprises a laser light emitting diode 532_(TX) while transistors 534 _(TX) and 544 _(TX) may comprise MMDT3946complementary NPN/PNP small signal transistors. Inverters 536 _(TX), 538_(TX) and 540 _(TX) may be furnished as portions of SN74LVC2G14 dualSchmitt-trigger inverters. Amplifier 542 _(TX) may also be provided asan FAN4931 rail-to-rail I/O CMOS amplifier device.

With reference additionally now to FIGS. 6A through 6G inclusive,greater structural detail of the representative device 400 of thepreceding figures is shown. With respect to FIGS. 6A and 6B inparticular, connection points for the serial wire debug (SWD) port 444and liquid crystal display (LCD) 446 (FIG. 4) are shown in addition tothe connection point for an LCD backlight. With particular reference toFIGS. 6C, 6D, 6E and 6F, the input/output (I/O) and other pinconnections to the microcontroller (MCU) 402 (FIG. 4) are shown. In theparticular representative embodiment of the present inventionillustrated and described, the MCU 402 may be conveniently furnished asa Kinetis K20 device available from Freescale Semiconductor, Inc. whichincludes on-board analog-to-digital (ADC) as well as digital-to-analog(DAC) converter and pulse width modulation (PWM) functionality.

With particular reference to FIG. 6G, the inclinometer 438 (FIG. 4) andassociated circuitry is shown in grater detail. The inclinometer 438 maybe conveniently provided as an LIS331 DLH MEMS digital output motionsensor available from STMicroelectronics, Inc. With reference also nowto FIG. 6H, the rate gyro 440 (FIG. 4) and associated components isillustrated. The rate gyro 440 may be furnished as an LPY503AL MEMSmotion sensor also available from STMicroelectronics, Inc.

Referring now to FIG. 6I, the keypad 436 (FIG. 4) is shown in schematicdetail in addition to a USB port 602 which couples to the K20 MCU 402previously described. Referring also now to FIG. 6J, the serial port 442is illustrated in greater detail and may comprise an ICL3221RS-232transmitter/receiver device available from Intersil Americas, Inc.

With reference additionally now to FIGS. 7A through 7E inclusive,greater structural detail of the representative device 400 of thepreceding figures is shown. With respect to FIG. 7A in particular, thefire switch 434 (FIG. 4) is shown along with some associated circuitry.The fire switch 434 initiates the transmission of laser signals from thephase transmission block 412 and pulse transmission block 448 whendepressed by a user of the device 400 once aimed at a target. Activecomponents illustrated include Schmitt-trigger devices 702, 704, 706,708 and 710 which may be furnished as 74LV132 quad 2-input NANDSchmitt-trigger devices available from NXP Semiconductors. Regulator 712may be supplied as an LM1117 low-dropout linear regulator available fromNational Semiconductor Corporation.

With reference now to FIG. 7B more detail as to certain aspects of thepower supply for the device 400 are shown including the connection for abattery in a portable implementation. Transistor 716 may be furnished asan IRLML6401 power MOSFET supplied by International RectifierCorporation. Switching converter 718 may be supplied as an NCP1421step-up DC-DC converter available from ON Semiconductor. Op amp 720 maybe implemented as an MCP6284 rail-to-rail op amp while transistor 722may be an MMBT2222 NPN general purpose amplifier device available fromFairchild Semiconductor Corporation.

Referring now to FIG. 7C, further portions of the power supply for thedevice 400 are shown. Inverters 724, 730 and 732 may be supplied asportions of 74AC14 hex inverters with Schmitt-trigger inputs alsoavailable from Fairchild Semiconductor Corporation. Device 726 may be a74VHC123 CMOS monostable multivibrator available from Toshiba AmericaElectronic Components while the diode array 728 may be furnished as aBAT54TW Schottky Barrier diode array available from Diodes, Inc. Op amp734 may be an MCP6284 rail-to-rail op amp while transistor 736 may befurnished as an MMBTA92 small signal PNP transistor available fromSTMicroelectronics, Inc. and transistor 738 may be an MMBTA42 NPN highvoltage amplifier device available from Fairchild SemiconductorCorporation.

With reference additionally now to FIG. 7D further details of the powersupply for the device 400 are shown including the charge pump. Device740 may also be furnished as a 74VHC123 CMOS monostable multivibrator aswas device 726. Inverters 742 may also be portions of 74AC14 hexinverters as were inverters 724, 730 and 732. Transistor 744 may besupplied as an IRLML0100TR power MOSFET available from InternationalRectifier Corporation. while diode arrays 746 and 748 may be supplied asBAS16VY triple high-speed switching diodes available from NXPSemiconductors. Op amp 750 may be supplied as an MCP6284 rail-to-rail opamp.

Referring additionally now to FIG. 7E still further details of the powersupply for the device 400 are shown. Device 752 may also be furnished asa 74VHC123 CMOS monostable multivibrator as was device 740 whileinverters 754 may also be portions of 74AC14 hex inverters as wereinverters 742. Transistor 756 may also be supplied as an IRLML0100TRpower MOSFET while op amps 758 and 760 may be an MCP6284 rail-to-rail opamp. Regulator 762 may be supplied as an NCP3985 micropower low-noisehigh power supply rejection ratio (PSRR) ultra-low dropout BiCMOSvoltage regulator available from On Semiconductor.

In operation, the phase portion of the system utilizes the fractional NPLL synthesis circuit 406 (FIG. 4) which allows for a direct downconversion from a high frequency of 230.0 MHz to the intermediatefrequency of 10.0 KHz. That combines with an analog switch 408 whichdirects the 230.0 MHz signal to either the phase transmission (PHASE_TX)block 412 which sends a laser beam out of the device 400 or the phasereference (PHASE_REF) block 410 which generates an internal referencebeam to the detector.

The fractional N PLL synthesis circuit 406 also generates a 230.01 MHzsignal, which provides the 10 kilohertz difference. The 230.01 MHzsignal is directed to an analog mixer, or down converter, 416. Thesignal which then is returned through the laser receiver 414 has afrequency of 230.0 MHz while the modulator receives the 230.01 MHzsignal. The difference frequency comes down through the low-pass filternetwork 418 as a 10 kilohertz signal and this frequency contains thephase information that was determined by the difference between theemitted beam, time of flight to the target and back. In this manner, thesame phase angle information that is being sought is preserved at a muchlower frequency which is then significantly more cost effective tomeasure and analyze where fractional hundredths of a degree areinvolved.

The whole system is also synchronized to the clock source 204 so thesampling can be synchronized precisely to that frequency. The MCU 402utilized in the particular implementation shown has an on-boardanalog-to-digital (ADC) conversion capability and all operations relateto the same system clock. In this regard, everything is then phasedlocked, so that when sampling is done to determine the phase angle,nothing can drift inasmuch as everything is linked to the common clocksource 204. Stated another way, the down conversion frequency and thedata sampling frequency are all phase-locked obviating the possibilityof drift over time and temperature that would otherwise requiresignificantly more expensive oscillators and controls in order toaccomplish a similar function. It should be noted that an external ADCcould also be used instead along with an appropriately synchronizedclock.

In purely phase-based laser systems multiple frequencies and multiplesteps are required to resolve the phase ambiguity between the emittedsignal and the return reflected signal. With the frequencies involved,the wave length of the signal is about 0.65 meters round trip so in themeasurement of the distance to a target on the order of 20 meters away,there would only be 30 whole cycles in which to measure the phase anglecycle. Accomplishing this measurement involves many additional steps andcircuit complexity to run different frequencies to resolve the phaseambiguity, all of which would take a significant amount of time awayfrom performing a high-accuracy measurement. By combining a phase-basedlaser system with a pulsed infrared system one can then much morequickly resolve the phase ambiguity as only one or two pulses of theinfrared is then accurate enough to determine exactly what the wholenumber of cycles are. This then allows for a much faster overallresolution plus the combined benefits of providing the optimizedhigh-frequency, short-range or high accuracy plus the slightly loweraccuracy for much longer range.

With respect to the pulsed infrared portion of the device 400, samplinggates 430 and 432 are high-speed analog switches. In operation, the MCU402 generates a TX_MATCH signal to flip-flop 424, a REF_MATCH signal toflip-flop 426 and a RX_MATCH signal to flip-flop 428. The timing of theports on the MCU 402 has approximately a +/−100 to 200 picoseconds ofuncertainty while what is required is closer to 10 picoseconds ofuncertainty or about 1.5 millimeters. Therefore, the port pins of theMCU cannot be utilized directly at the very high levels of accuracy orresolution being sought. The flip-flops 424, 426 and 428 are thenutilized as synchronizers.

The sequence of events for a measurement begins with the initiation ofthe TX_MATCH signal at a nominal time T₀ (Tzero) through flip-flop 424to the programmable delay circuit 420 with the V_DEL signal coming froma digital-to-analog converter (DAC) onboard the MCU 402. In operation,the V_DEL signal creates a ramp and it should be noted that,alternatively, the onboard DAC could also be an external DAC. The outputof the delay circuit 420 is switched through demultiplexer 422 to firethe laser pulse and to form a calibration reference through buffer 516(FIG. 5B) for sampling gates 430 and 432.

The delay circuit 420 is calibrated so that one clock period can bedetermined and, in calibration mode using the REF/CAL channel, the delayis moved around until the REF_MATCH signal is moved by exactly one clockperiod. This enables the determination of the voltage range betweenV_DEL high and V_DEL low with the latter being the clock period minusone. The higher the voltage on V_DEL, the greater the delay so the clockmust be increased to make it match. Operating V_DEL between its high andlow positions enables the determination that the delay circuit 420 haschanged delay by exactly one clock period and this same operation isperformed on the reference channel for REF_MATCH and the receiverchannel for RX_MATCH. Since a common signal is applied to both, anysmall temperature differences between the two channels is factored out.In TX mode, the laser pulse is fired toward the target and the laserpulse generates a fixed firing point or reference signal to samplinggate 430. The reference signal TX REF is taken from the actual firecircuit and represents the exact point at which the laser diode fires.

The biasing network employed in the device 400 ensures that the signalsremain within the correct dynamic range. Causing sampling gate 430 toenter the reference mode, and by adjusting the reference match REF_MATCHcount and the level of V_DEL, it is possible to measure exactly when thelaser fired. Once the laser is fired with the pulse being emitted, wenow have an exact time which, by monitoring the firing, varying thedelay, firing off a sequence of laser pulses and varying the delay, itcan be determined exactly when the laser pulse fired match. Exactly thesame thing can then be done with the receive signal. The receiver isplaced into RX mode, and RX_MATCH and V_DEL are adjusted to measure theRX sample and find exactly when the signal was returned from the target.This provides a whole clock number for a REF_MATCH plus the delaysetting in a fractional count of the clock, which represents the pointin time when the laser pulse was emitted. We now have an exact match forthe receiver in terms of whole clock periods and a fractional part whenthe laser pulse was sent to the target, the difference being the actualflight time between transmission and reception. In this manner it ispossible to calibrate out all the different temperature coefficientsassociated with the delay circuit 420, transmit delays and the samplinggates.

The MCU 402 employed in the representative embodiment of the presentinvention disclosed comprises the on-board DACs, the on-board ADCs andprovides the on-board PWM. Keypad 436 input is shown in FIG. 6I andthere are also provided inputs for the tilt sensor 438 (FIG. 6G) and arate gyro 440 (FIG. 6H). While a compass might also be included as apart of the device 400 such may not be as effective as rate gyro 440 inthe area of construction sites and other places contaminated withrelatively large ferrous objects.

With particular reference to FIG. 7B, switching converter 718 is theprimary device which regulates boost from the battery. Additional powersupply components include Schmitt-trigger devices 704, 706, 708 and 710(FIG. 7A) which are used for power on/off and fire switch detection andvarious power supplies. Regulator 762 (FIG. 7E) is employed as a 3.3volt post regulator because the logic components of the device 400operate at 3.3 volts. A separate discrete device 1.8 volt regulatorcomprising op amp 720 and transistor 722 (FIG. 7B) is also utilizedwhich provides the core voltage for the clock oscillator chip 522 aswell as other components.

The TX circuit has an extra line (TX_BIAS; FIG. 7C) which supplies anintermediate voltage of approximately 7.5 volts generated from a 4 voltrail. This allows the TX circuit itself to run faster. For the basictransmission high voltage, a simple inductor flyback booster circuit isprovided comprising device 752 (FIG. 7E) which generates control pulses.Inverters 754 boost up the drive level while transistor 756 and theassociated inductor comprise the flyback booster circuit with diodearray 748 forming the output diode. The TX power is controlled by pulsewidth modulation coming in on TX_PWR signal (FIG. 7E) from the MCU 402which is proportional to TX_HV. The receiver utilizes the same device740 (FIG. 7D) with a fixed pulse width and 3-stage diode charge pump onthe output of the flyback converter to achieve 300 volts. A feedbackloop comprises two resistors in conjunction with op amp 750 to controlthe switching to generate 300 volts raw for the RX. A post-linearregulator comprising transistors 736 and 738 with op amp 734 (FIG. 7C)regulates the 300 volts down to substantially between 200 volts to 280volts which is a typical bias range. The PWM scheme is set up to provideapproximately 1 in 300 resolution, or just 1 volt roughly at 300 volts.However, the 1 volt is then split about 300 levels finer, or on theorder of 30 millivolts, to provide very fine control for the particulartype of avalanche photodiode (APD) which might be utilized in a tapelaser which is a very sharp breakdown device. For that reason the biaspoint must be held within a fraction of a volt in order to achieveproper operation.

Overall, a range-finding instrument employing the system and method ofthe present invention provides significantly improved measurementresolution, at low cost with a greater distance measurement capability.Further, it can be employed in dusty or industrial environments wherethe transmitted CW beam becomes obscured or is otherwise interfered withon a regular basis.

While there have been described above the principles of the presentinvention in conjunction with specific apparatus, it is to be clearlyunderstood that the foregoing description is made only by way of exampleand not as a limitation to the scope of the invention. Particularly, itis recognized that the teachings of the foregoing disclosure willsuggest other modifications to those persons skilled in the relevantart. Such modifications may involve other features which are alreadyknown per se and which may be used instead of or in addition to featuresalready described herein. Although claims have been formulated in thisapplication to particular combinations of features, it should beunderstood that the scope of the disclosure herein also includes anynovel feature or any novel combination of features disclosed eitherexplicitly or implicitly or any generalization or modification thereofwhich would be apparent to persons skilled in the relevant art, whetheror not such relates to the same invention as presently claimed in anyclaim and whether or not it mitigates any or all of the same technicalproblems as confronted by the present invention. The applicants herebyreserve the right to formulate new claims to such features and/orcombinations of such features during the prosecution of the presentapplication or of any further application derived therefrom.

As used herein, the terms “comprises”, “comprising”, or any othervariation thereof, are intended to cover a non-exclusive inclusion, suchthat a process, method, article, or apparatus that comprises arecitation of certain elements does not necessarily include only thoseelements but may include other elements not expressly recited orinherent to such process, method, article or apparatus. None of thedescription in the present application should be read as implying thatany particular element, step, or function is an essential element whichmust be included in the claim scope and THE SCOPE OF THE PATENTEDSUBJECT MATTER IS DEFINED ONLY BY THE CLAIMS AS ALLOWED. Moreover, noneof the appended claims are intended to invoke paragraph six of 35 U.S.C.Sect. 112 unless the exact phrase “means for” is employed and isfollowed by a participle.

What is claimed is:
 1. A rangefinding instrument comprising: apulse-based signal transmission system comprising an infrared laserdiode; a phase-based signal transmission system comprising a visiblelight emitting source; and at least one distance computing circuitcoupled to said pulse-based and phase-based transmission systemsoperative to determine a range to a target based upon a time of flightof said pulse-based signals and a phase shift of said phase-basedsignals reflected from said target, wherein an indicated range of saidinstrument to said target is based upon a more accurate range to saidtarget as determined by said at least one computing circuit based uponeither said phase-based signals or said pulse-based signals.
 2. Therangefinding instrument of claim 1 wherein said visible light emittingsource is modulated to produce a continuous wave signal output.
 3. Therangefinding instrument of claim 2 wherein said continuous wave signaloutput is gated “on” and “off” to produce a discreet series ofcontinuous wave signals.
 4. The rangefinding instrument of claim 1wherein said at least one distance computing circuit detects anexistence of interfering material in determining said range to saidtarget based upon said phase shift of said phase-based signals reflectedfrom said target and computes said range to said target based upon atime of flight of said pulse-based signals.
 5. The rangefindinginstrument of claim 1 wherein said at least one distance computingcircuit further comprises: a detector for receiving said pulse-basedsignals and said phase-based signals reflected from said target.
 6. Arangefinding instrument comprising: a pulse-based signal transmissionsystem comprising an infrared laser diode; a phase-based signaltransmission system comprising a visible light emitting source; and atleast one distance computing circuit coupled to said pulse-based andphase-based transmission systems operative to determine a range to atarget based upon a time of flight of said pulse-based signals and aphase shift of said phase-based signals reflected from said target,wherein said at least one distance computing circuit computes said rangeto said target based upon said phase shift of said phase-based signalsreflected from said target and provides calibration information forcomputation of said range to said target based upon a time of flight ofsaid pulse-based signals.
 7. The rangefinding instrument of claim 6wherein said visible light emitting source is modulated to produce acontinuous wave signal output.
 8. The rangefinding instrument of claim 7wherein said continuous wave signal output is gated “on” and “off” toproduce a discreet series of continuous wave signals.
 9. Therangefinding instrument of claim 6 wherein said at least one distancecomputing circuit detects an existence of interfering material indetermining said range to said target based upon said phase shift ofsaid phase-based signals reflected from said target and computes saidrange to said target based upon a time of flight of said pulse-basedsignals.
 10. The rangefinding instrument of claim 6 wherein said atleast one distance computing circuit further comprises: a detector forreceiving said pulse-based signals and said phase-based signalsreflected from said target.
 11. A method for determining range to atarget comprising: transmitting a pulse-based signal to said target froman infrared laser diode; transmitting a phase-based signal to saidtarget from a visible light emitting source; determining a range to saidtarget based upon a time of flight of said pulse-based signal and aphase shift of said phase-based signal reflected from said target; anddisplaying said range to said target based upon a determination of whichof said pulse-based or phase-based signals is considered to be mostaccurate.
 12. The method of claim 11 wherein said transmitting saidphase-based signal to said target is carried out by: modulating saidvisible light emitting source.
 13. The method of claim 12 furthercomprising: gating said modulated visible light emitting source “off”and “on”.
 14. The method of claim 11 further comprising: detecting anexistence of interfering material in determining said range to saidtarget based upon said phase shift of said phase-based signals reflectedfrom said target; and computing said range to said target based upon atime of flight of said pulse-based signals.
 15. The method of claim 11further comprising: providing a detector; and receiving said pulse-basedsignals and said phase-based signals reflected from said target withsaid detector.
 16. A method for measuring distance comprising: emittinga continuous wave laser beam from a visible light emitting source towarda target; emitting a pulsed laser beam from an infrared laser diodetoward said target; receiving reflections of said continuous wave laserbeam and said pulsed laser beam from said target; computing a distanceto said target based on each of said emitted and reflected continuouswave and pulsed laser beams; determining which of said computeddistances is more accurate; and indicating a range to said target basedupon said more accurate range as determined by at least one computingcircuit.
 17. A rangefinding instrument comprising: a pulse-based signaltransmission system; a phase-based signal transmission system; and atleast one distance computing circuit coupled to said pulse-based andphase-based transmission systems operative to determine a range to atarget based upon a time of flight of said pulse-based signals and aphase shift of said phase-based signals reflected from said target,wherein an indicated range of said instrument to said target is basedupon a more accurate range to said target as determined by said at leastone computing circuit based upon either said phase-based signals or saidpulse-based signals and wherein said at least one distance computingcircuit detects an existence of interfering material in determining saidrange to said target based upon said phase shift of said phase-basedsignals reflected from said target and computes said range to saidtarget based upon a time of flight of said pulse-based signals, said atleast one distance computing circuit further identifying said existenceof said interfering material to a user of said rangefinding instrumenton a display.
 18. A rangefinding instrument comprising: a pulse-basedsignal transmission system; a phase-based signal transmission system;and at least one distance computing circuit coupled to said pulse-basedand phase-based transmission systems operative to determine a range to atarget based upon a time of flight of said pulse-based signals and aphase shift of said phase-based signals reflected from said target,wherein said at least one distance computing circuit computes said rangeto said target based upon said phase shift of said phase-based signalsreflected from said target and provides calibration information forcomputation of said range to said target based upon a time of flight ofsaid pulse-based signals and wherein said at least one distancecomputing circuit detects an existence of interfering material indetermining said range to said target based upon said phase shift ofsaid phase-based signals reflected from said target and computes saidrange to said target based upon a time of flight of said pulse-basedsignals, said at least one distance computing circuit furtheridentifying said existence of said interfering material to a user ofsaid rangefinding instrument on a display.
 19. A method for determiningrange to a target comprising: transmitting a pulse-based signal to saidtarget from a laser diode; transmitting a phase-based signal to saidtarget; determining a range to said target based upon a time of flightof said pulse-based signal and a phase shift of said phase-based signalreflected from said target; displaying said range to said target basedupon a determination of which of said pulse-based or phase-based signalsis considered to be most accurate; detecting an existence of interferingmaterial in determining said range to said target based upon said phaseshift of said phase-based signals reflected from said target; computingsaid range to said target based upon a time of flight of saidpulse-based signals; and identifying said existence of said interferingmaterial to a user of said rangefinding instrument on a display.